Numerous types of malfunctions and abnormalities that commonly occur in the cardiovascular system, if not diagnosed and appropriately treated or remedied, will progressively decrease the body's ability to supply sufficient oxygen to satisfy the coronary oxygen demand when the individual encounters stress. The progressive decline in the cardiovascular system's ability to supply oxygen under stress conditions will ultimately culminate in a heart attack, i.e., myocardial infarction event that is caused by the interruption of blood flow through the heart resulting in oxygen starvation of the heart muscle tissue (i.e., myocardium). In serious cases, the consequences are mortality while in less serious cases, permanent damage will occur to the cells comprising the myocardiurn that will subsequently predispose the individual's susceptibility to additional myocardial infarction events.
In addition to potential malfunctions and abnormalities associated with the heart muscle and valve tissues (e.g., hypertrophy), the decreased supply of blood flow and oxygen supply to the heart are often secondary symptoms of debilitation and/or deterioration of the blood flow and supply system caused by physical and biochemical stresses. While some of these stresses are unavoidable, e.g., increasing age, heredity and gender, many of the causative factors of cardiovascular diseases and malfunction are manageable, modifiable and treatable if their debilitating effects on the cardiovascular system are detected early enough. Examples of such modifiable risk factors include high blood pressure, management of blood cholesterol levels, Diabetes mellitus, physical inactivity, obesity, stress, and smoking. Examples of cardiovascular diseases that are directly affected by these types of stresses include atherosclerosis, coronary artery disease, peripheral vascular disease and peripheral artery disease. In many patients, the first symptom of ischemic heart disease (IHD) is myocardial infarction or sudden death, with no preceding chest pain as a warning.
Screening tests are of particular importance for patients with risk factors for IHD. The most common initial screening test for IHD is to measure the electrical activity over a period of time which is reproduced as a repeating wave pattern, commonly referred to as an electrocardiograph (ECG), showing the rhythmic depolarization and repolarization of the heart muscles. Analysis of the various waves and normal vectors of depolarization and repolarization yields important diagnostic information. However, ECG measurements are not particularly sensitive nor are the data very useful for detecting cardiovascular abnormalities or malfunctions. Therefore, stressing the heart under controlled conditions and measuring changes in the ECG data is usually, but not always, the next step. The stresses may be applied by the performance of physical exercise or alternatively, by administration of pharmaceutical compounds such as dobutamine, which mimic the physiological effects of exercise. Other screening tests for IHD include the radionucleotide stress test which involves injecting a radioactive isotope (typically thallium or cardiolyte) into a patient's bloodstream, then visualizing the spreading of the radionucleotide throughout the vascular system and its absorption into the heart musculature. The patient then undergoes a period of physical exercise after which, the imaging is repeated to visualize changes in distribution of the radionucleotide throughout the vascular system and the heart. Stress echocardiography involves ultrasound visualization of the heart before, during and after physical exercise. The radionucleotide stress test and stress echocardiography are often used in combination with ECG measurements in order to gain a clearer understanding of the state of individual's cardiovascular health.
However, there are a number of serious limitations associated with the use of ECG and related stress tests for detecting abnormalities and malfunctions that are indicators of ischemic heart disease. ECG printouts provide a static record of a patient's cardiovascular function at the time the testing was done, and may not reflect severe underlying heart problems at a time when the patient is not having any symptoms. The most common example of this is in a patient with a history of intermittent chest pain due to severe underlying coronary artery disease. This patient may have an entirely normal ECG at a time when he is not experiencing any symptoms despite the presence of an underlying cardiac condition that normally would be reflected in the ECG. In such instances, the ECG as recorded during an exercise stress test may or may not reflect an underlying abnormality while the ECG taken at rest may be normal. Furthermore, many abnormal patterns on an ECG may be non-specific, meaning that they may be observed with a variety of different conditions. They may even be a normal variant and not reflect any abnormality at all. Routine exercise ECG is not recommended in patients who have no signs or symptoms of coronary artery disease. Exercise ECG is notoriously ineffective at predicting underlying coronary artery disease, and a positive exercise ECG test in an apparently healthy patient is not known to have any association with cardiovascular morbidity and mortality.
Ballistocardiography (BCG) is a non-invasive method of graphically recording minute movements on an individual's body surface as a consequence of the ballistic i.e., seismic forces associated with cardiac function, e.g., myocardial contractions and related subsequent ejections of blood, ventricular filling, acceleration, and deceleration of blood flow through the great vessels. These minute movements are amplified and translated by a pick-up device (e.g., an accelerometer) placed onto a patient's sternum, into signals with electrical potentials in the 1-20 Hz frequency range and recorded on moving chart paper. The rhythmic contractions of the heart and related flows of blood within and from the heart's chambers under resting and stressed conditions produce repeating BCG wave patterns that enable visual detection and assessment by qualified diagnosticians of normal and abnormal cardiovascular function. The BCG records the vigor of cardiac ejection and the speed of diastolic filling. It provides a practical means of studying the physiologic response of the heart in its adjustment to the stress of exercise. The application of the light BCG exercise test to subjects without clinical or ECG evidence of heart disease, or to hypertensive subjects, or to patients with coronary artery disease and to those suspected of having myocarditis, provides information of importance which cannot be obtained from any other means of physical diagnosis or from the BCG at rest (Mandelbaum et al., 1954. Circulation 9:388-399). The most common BCG wave pattern classification system is known as the Starr system (Starr et al., 1961, Circulation 23: 714-732) and identifies four categories of cardiovascular function depending on the abnormalities in the measured BCG signals. In class 1, all BCG complexes are normal in contour. In class 2, the majority of the complexes are normal, but one or two of the smaller complexes of each respiratory cycle are abnormal in contour. In class 3, the majority of the complexes are abnormal in contour, usually only a few of the largest complexes of each respiratory cycle remaining normal and in class 4, there is such complete distortion that the waves cannot be identified with confidence, and the onset of ejection could not be located without the assistance of a simultaneous ECG (Starr, 1964, J. Am. Med. Assoc. 187:511). In general, a normal healthy person should belong to Starr class 1, and person belonging to class 3 or 4 has a significant abnormality in one or more components of the cardiovascular system. However, the classification is not exact, as it is done visually and depends on the person making the classification (Starr, 1964, J. Am. Med. Assoc. 187:511).
Coronary angiography enables visualization and assessment of potential cardiovascular abnormalities and malfunctions that are not possible to detect with the afore-mentioned stress tests, including as occlusions, stenosis, restenosis, thrombosis, aneurismal enlargement of coronary artery lumens, heart chamber size, heart muscle contraction performance and heart valve function. During a coronary angiogram, a small catheter is inserted through the skin into an artery in either the groin or the arm. Guided with the assistance of a fluoroscope, the catheter is then advanced to the opening of the coronary arteries, the blood vessels supplying blood to the heart. Next, a small amount of radiographic contrast solution is injected into each coronary artery. The images that are produced are called the angiogram. Although angiographic images accurately reveal the extent and severity of all coronary arterial blockages and details of the heart musculature, the procedure is invasive and requires the use of local anesthesia and intravenous sedation.